1. Field of the Invention
This invention relates to a purifying apparatus possessing a vapor dispersing device such mechanism as to deflect a current of vapor to a specific direction or a purifying apparatus composed of a distillation column and a re-boiler. More particularly, the invention relates to a distillation column and a purifying apparatus which provide distillation and purification of an easily polymerizable compound.
2. Description of Related Art
Distillation is a process which is adopted predominantly for purifying a volatile substance. When the vapor abounding in a low boiling component and ascending to the top of a distillation column is cooled and condensed and the resultant condensate is made to descend from the top of the column and contact the vapor rising from below, the low boiling component in the condensate is gasified and, at the same time, the high boiling component in the vapor is aggregated and liquefied. Consequently, the low boiling component of the vapor departing from the top of the column gains in purity and the high boiling component of the liquid falling to the bottom of the column likewise gains in purity. This refluxing operation constitutes the rectification. Generally, it is in a distillation column that this rectification is carried out.
In this distillation column, the gas-liquid contact for the rectification is implemented by means of trays and a packing disposed in the column. Generally, the liquid supplied from the upper part of the column and the vapor supplied from the lower part of the column are brought into the gas-liquid contact in a cruciform flow or counter flow on the trays and on the surface of the packing eventually to effect gas-liquid mass transfer. It is known that in the gas-liquid mass transfer operation proceeding in the distillation column, the product of this operation is deteriorated by the inferiority of the gas-liquid contact and the inclusion of the bottom liquid of the distillation column in the product. When the compound for distillation happens to be an easily polymerizable compound, the formation of a polymer degrades the efficiency of separation and purification and deteriorates the quality of the product of distillation and the polymer itself adheres to the apparatus and blocks the flow paths for the vapor and the liquid possibly to the extent of obstructing continuous operation of the apparatus.
When the gas phase introduced into the distillation column is drifted only in a limited part, for example, the gas-liquid contact is not sufficiently effected and the efficiency of separation and purification is degraded. Particularly, when the easily polymerizable compound is distilled, it forms a polymer and the polymer adheres to and clogs the various devices used in the process of purification, with the result that the efficiency of separation and purification will be degraded and the quality of the product will be deteriorated. The process, therefore, demands early removal of the polymer. The removal of this polymer compels the process of purification to stop and the removal of adhering polymer is generally difficult.
The degradation of the efficiency of separation and purification and the polymerization mentioned above are readily caused by the drift which is generated in the bottom part of the distillation column. The distillation column generally incorporates therein a re-boiler for gasifying part of the bottom liquid of the column and a re-boiler vapor inlet part for forwarding the produced vapor into the interior of the column for the purpose of heating the bottom of the column. When the vapor blown out of this re-boiler is immediately supplied from the lateral side of the column without being rectified via the nozzle of the inlet part, the vapor in the distillation column is drifted only in part of the cross section of the column by virtue of the propelling force thereof tending upward. Further, in consequence of the upward drift, the liquid flowing down from the upper part of the distillation column is likewise drifted and the gas-liquid contact is rendered deficient. As a result, the efficiency of separation in the distillation column is lowered and the yield of product is degraded. Further, the drift of the vapor gives rise to a vapor stagnating part inside the distillation column and the condensation of the easily polymerizable compound in the gas phase induces the compound to form a polymer easily.
From the re-boiler, the bottom liquid of the column circulated via the re-boiler is copiously blown in besides the vapor. As this vapor ascends at the prevalent speed inside the distillation column, it entrains splashed droplets and the liquid containing the bottom liquid of the column is carried to the trays in the upper stages together with the vapor. To be specific, the efficiency of separation by distillation is degraded because the vapor flowing to the upper part of the distillation column entrains the splashed droplets of the bottom liquid of the column and part of the bottom liquid of the column is suffered to mingle into the product of distillation at the top of the column.
Particularly where the object of distillation is such an easily polymerizable compound as acrylic acid or a product of the esterification thereof, the polymerization due to the drift tends to occur easily. Acrylic acid, for example, is produced by catalytic gas phase oxidation of propylene and/or acrolein with a molecular oxygen-containing gas. The acrylic acid-containing liquid consequently formed is distilled in the distillation column. Since this acrylic acid-containing liquid has a composition containing such impurities as water, acetic acid, and acrolein, the polymerization of acrylic acid occurs very easily. The polymerization of (meth)acrylic acid, therefore, is repressed by adding various polymerization initiators such as hydroquinone, methoquinone, and phenothiazine either singly or in the form of a mixture of two or more members to the process of production. When the vapor containing acrylic acid is led from the re-boiler to the distillation column, however, since the polymerization inhibitor exists only in the liquid phase, the vapor or the condensed liquid from the gas phase part contains substantially no polymerization inhibitor and, on being heated, easily undergoes polymerization. Particularly, the condensation in the gas phase part tends to occur in the vapor stagnating part which is generated by the drift of vapor inside the column and, as a result, induces polymerization in the column.
As a method for preventing the splashed droplets from being entrained by the vapor in the distillation column, JP-A-03-196,801 discloses an invention which, in an operation for flash distillation, aims to accomplish this prevention by providing the distillation column at the center thereof with an umbrella baffle plate separated from the inner wall of the distillation column. JP-A-51-2,675 likewise discloses a method for preventing a quenching column from clogging a nozzle led to the center of the quenching column by depriving the quenching column of a plate for collision and dispersion of gas and causing the nozzle to extend obliquely downward in the direction of the central part of the bottom liquid surface of the column and eventually join the interior of the column instead of thrusting through the wall of the column. The polymerization, therefore, is prevented by excluding the plate for collision and dispersion of gas thereby limiting the portion giving rise to a temperature gradient from the temperature of the quenching gas to the temperature of the condensed liquid only to the part of the wall of the column that surrounds the base of the nozzle. This invention has originated in the light of the fact that when the vapor spouted through a gas blowing nozzle thrust to the center of a quenching column is a polymerizing unsaturated compound and the neighborhood of the nozzle is not suddenly cooled fully satisfactorily, a high boiling substance adheres to the inner wall of the nozzle, gains in growth, and clogs the nozzle again and again.
Unfortunately, the method disclosed in JP-A-51-2,675 mentioned above suffers the condensation of vapor phase substances by contacting with the nozzle, though it is inherently capable of preventing the nozzle from being clogged. Further, by merely discharging the reaction gas via the nozzle in the direction of the bottom liquid in the column, the splashed droplets cannot be prevented from being entrained by the vapor fully satisfactorily and the efficiency of separation and purification cannot be exalted fully satisfactorily.
This invention aims to prevent the vapor from drifting in the distillation column, improve the efficiency of distillation, and prevent the vapor from undergoing polymerization as well. It also aims to inhibit the bottom liquid of the column from entraining splashed droplets and improve the efficiency of distillation.
The inventors, after pursuing a study on distillation columns, have discovered that, by disposing a vapor dispersing apparatus furnished with an opening part of a specific area directed downward from the horizontal direction at the vapor inlet part of a re-boiler, the drift of the vapor from the re-boiler is repressed, the efficiency of separation in the distillation column is improved, and the polymerization of the vapor inside the column is inhibited. This invention has been perfected as a result.
The problems enumerated above are accomplished by the following items (1)-(10).
(1) A purifying apparatus characterized by being equipped with a vapor dispersing device positioned in a vapor inlet part of the apparatus on the lateral inside thereof an opening part facing downward from the horizontal direction and a horizontal projected cross section area in the range of 10-40% of the cross section area of the apparatus.
(2) A purifying apparatus according to the item (1) mentioned above, wherein the distance from the opening part of the vapor dispersing device to the liquid level in the bottom part is in the range of 0.1-5 times the diameter of the vapor inlet part.
(3) A purifying apparatus according to the item (1) mentioned above, wherein the purifying apparatus is a plate tower or a packed tower and the distance from the opening part of the vapor dispersing device to the lowermost tray or a packing supporting member is in the range of 1.5-8 times the diameter of the vapor inlet part.
(4) A purifying apparatus according to the item (1) mentioned above, wherein the apparatus is provided with not less than two vapor dispersing devices or the vapor dispersing device is provided with not less than two opening parts.
(5) A purifying apparatus according to the item (1) mentioned above, wherein the vapor dispersing device is provided in the upper part thereof with a hole for releasing a vapor and the total hole area is in the range of 0.001-2% of the cross section area of the apparatus.
(6) A purifying apparatus possessed of a distillation column and a re-boiler disposed outside the column, characterized by being equipped with a vapor outlet nozzle of the re-boiler connected to the column side wall in the gas phase part of the bottom of the distillation column and a vapor dispersing device positioned in a vapor inlet part owned by the apparatus on the lateral inside thereof an opening part facing downward from the horizontal direction and a horizontal projected cross section area in the range of 10-40% of the cross section area of said apparatus.
(7) A purifying apparatus according to the item (6) mentioned above, wherein the distillation column is a purifying apparatus set forth in any of the items (2)-(5) mentioned above and equipped with a molecular oxygen supplying part.
(8) A distillation column according to any of the items (1)-(5) mentioned above, characterized by being used for distilling an easily polymerizable compound.
(9) A distillation column according to the item (8) mentioned above, wherein the easily polymerizable compound is (meth)acrylic acid and/or an ester thereof.
(10) A distillation column, characterized by being equipped on the inner wall of a distillation column with a mechanism for deflecting the re-boiler vapor introduced into the distillation column downward from the horizontal direction.
The installation of the vapor dispersing device in the re-boiler vapor outlet results in endowing the distillation column with improved efficiency of separation because the device is capable of repressing the drift of the vapor in the column and affording fully satisfactory gas-liquid contact.
This installation also brings about such effects as curbing the drift of the vapor in the column, decreasing the vapor stagnating part in the column, and allaying the condensation of the vapor and thereby inhibiting the occurrence of a polymer.
The formation of polymer may be further repressed by providing the distillation column or the purifying apparatus with a molecular oxygen supplying part.